Microearthquake seismicity and focal mechanisms in and around Gyeongju, southeastern Korean Peninsula

Abstract:

Spatiotemporal distribution and focal mechanism of microearthquake are important to understand the correlation between stress, seismic activity and earth structure in and around hypocenter. Three moderate earthquakes (M_W 5.1, 5.5 and 4.4) occurred in September 2016 in Gyeongju located in the southeastern part of the Korean Peninsula and multitudinous aftershocks have continued to the present day. For scrutiny about the Gyeongju earthquake sequence, a dense seismic network was constructed in the range of 40 * 40 km² centered the Gyeongju earthquake sequence. We analyzed the microearthquake seismicity and its focal mechanisms to understand the seismic and geological characteristics in and around Gyeongju. The earthquake detection was performed through auto-picking based on the Short-Term Average versus Long-Term Average (STA/LTA) and Akaike Information Criterion (AIC) methods. The focal mechanisms of detected earthquakes were determined from P-wave first-motion polarity. The detected epicenters are mainly distributed in some region: the Gyeongju earthquake epicenter, eastern part of Ulsan fault, Pohang earthquake epicenter, and quarries located in and around the seismic network. Especially, the earthquakes occurred most frequently at the Gyeongju earthquake epicenter and eastern part of Ulsan fault. The focal depths of the earthquakes occurred eastern part of the Ulsan fault are distributed about 10 km, and those focal mechanisms are mainly determined as reverse-fault. In case of the Gyeongju earthquake epicenter, the range of the focal depths is from 12 km to 16 km, and the focal mechanisms are dominantly determined as strike-slip fault. The frequency of earthquakes occurrence is considerably reduced compared to immediately after the Gyeongju earthquake. However, it is confirmed that the geometry of the causative fault of the Gyeongju earthquake sequence have not changed significantly.